[PATCH v6 net-next 09/14] tcp: add TCP_SYNACK_RETRANS synack_type

chia-yu.chang
2025-11-14T07:14:15Z

Before this patch, retransmitted SYN/ACK did not have a specific synack_type;
however, the upcoming patch needs to distinguish between retransmitted and
non-retransmitted SYN/ACK for AccECN negotiation to transmit the fallback
SYN/ACK during AccECN negotiation. Therefore, this patch introduces a new
synack_type (TCP_SYNACK_RETRANS).

Signed-off-by: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>

---
v6:
- Add new synack_type instead of moving the increment of num_retran.
---
include/net/tcp.h | 1 +
net/ipv4/tcp_output.c | 3 ++-
2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/net/tcp.h b/include/net/tcp.h
index a8eb67ff1568..510d2e595b08 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -541,6 +541,7 @@ enum tcp_synack_type {
TCP_SYNACK_NORMAL,
TCP_SYNACK_FASTOPEN,
TCP_SYNACK_COOKIE,
+ TCP_SYNACK_RETRANS,
};
struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst,
struct request_sock *req,
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 8039c726d235..5fa14a73d03f 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -3921,6 +3921,7 @@ struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst,

switch (synack_type) {
case TCP_SYNACK_NORMAL:
+ case TCP_SYNACK_RETRANS:
skb_set_owner_edemux(skb, req_to_sk(req));
break;
case TCP_SYNACK_COOKIE:
@@ -4606,7 +4607,7 @@ int tcp_rtx_synack(const struct sock *sk, struct request_sock *req)
/* Paired with WRITE_ONCE() in sock_setsockopt() */
if (READ_ONCE(sk->sk_txrehash) == SOCK_TXREHASH_ENABLED)
WRITE_ONCE(tcp_rsk(req)->txhash, net_tx_rndhash());
- res = af_ops->send_synack(sk, NULL, &fl, req, NULL, TCP_SYNACK_NORMAL,
+ res = af_ops->send_synack(sk, NULL, &fl, req, NULL, TCP_SYNACK_RETRANS,
NULL);
if (!res) {
TCP_INC_STATS(sock_net(sk), TCP_MIB_RETRANSSEGS);
--
2.34.1